1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to network echo cancellation of echoes originating from a mobile terminal, and especially a digital mobile phone.
2. Description of Related Art
"Echo" is a phenomenon that can occur in a telephony system whenever a portion of transmitted speech signal energy is reflected back to the source of the signal. As such, echo has been a problem in telephone networks for a very long time. There can be many different sources of echo, such as, for example, a hybrid circuit that converts a 4-wire line to a 2-wire line in a Public Switched Telephone Network (PSTN) subscriber interface, or acoustical cross-talk in a mobile radiotelephone. A considerable amount of echo can be generated if a telephone set is used in the hands-free mode (e.g., in teleconferencing systems or automobiles).
Echo may not be a problem if the round-trip delay associated with the echo is relatively small. However, the presence of echo along with a substantial delay (e.g., on the order of hundreds of milliseconds) can severely degrade the quality of speech signals, and thus become quite annoying for a listener. The delay in a telephone system can be a physical delay (e.g., caused by the long transmission path over a satellite link) or processing delay. Processing delay is common in digital mobile applications.
An echo canceller is a device that is commonly used in telephony systems to suppress or remove echoes in long distance traffic. For example, in cellular Public Land Mobile Networks (PLMNs), echo cancellers are used in mobile services switching centers (MSCs) to suppress or remove echoes in speech traffic. However, these echo cancellers are designed to cancel echoes originating only in the PSTN side of the connection.
In principle, a digital mobile radiotelephone handset should not generate echoes, because the connection used comprises 4-wires down to the handset. In practice, however, many such mobile phones generate echoes that originate from acoustical or mechanical cross-talk in the handset. This type of echo is annoying to users, especially if the system operator has elected to raise the downlink signal levels. Raising the downlink signal levels has become an accepted practice, since many customers have complained about the low output levels from the mobile handsets' speakers.
There are a number of apparent differences between the characteristics of the echo signals that originate in PSTNs and those that originate in digital mobile phones. For example, the echo path from a PSTN is quasi-linear and, therefore, can be readily modeled by a linear filter. A number of existing solutions, which are based on adaptive filtering techniques, can be used successfully to cancel these types of echoes. As such, the echo return loss (ERL) from a PSTN depends on the balance circuitry used in the network. As a general rule, the ERL (measured in dB) can be considered as a random variable selected from a Gaussian distribution, with a mean of 13.6 dB and a standard deviation of 2.8 dB for a segregated loop balancing scheme.
On the other hand, the echo path for a digital mobile phone is non-linear and time-varying, due to the use of two speech coder/decoder (codec) pairs and radio channels in the transmission path. Additionally, the level of the echo in a digital mobile phone is much lower from that of a PSTN. For example, the specification for the digital cellular Global System for Mobile Communications (GSM) requires an ERL of 46 dB (for the mobile phones) measured for pure tones of level 0 dBm in the 300-3400 Hz band. However, the ERL can be lower if signals other than pure tones are used for the measurements but suppression levels of about 40 dB can still be expected. In fact, the ERL from a digital mobile phone is comparable to the ERL that can be obtained by the linear filter portion of a conventional PSTN echo canceller. However, an effect of the extensive signal processing in the mobile echo path is that the delay between the signal and the echo is longer than in the PSTN echo case.
A natural approach to controlling echoes originating from a mobile handset is to include an echo canceller in the handset. Another approach would be to use a conventional network echo canceller with the echo cancellation directed toward the mobile subscriber. However, a problem with these approaches is that it is impractical and expensive to refurbish existing mobile terminals. Also, an imposition of tighter echo control requirements for digital mobile terminals would influence future products only. As such, tighter echo control requirements for such terminals would not improve the speech quality for the terminals already in use. Furthermore, a conventional network echo canceller will fail if used to cancel echoes originating from a digital mobile terminal, because the resulting echo path is significantly different from the echo path that a network echo canceller is designed to handle. Consequently, new solutions are needed to resolve the problems associated with echoes originating from digital mobile terminals. As described below, the present invention successfully resolves these problems.